Prime Minister John Key's collapse at a Christchurch restaurant last night may have been due to jet lag.

Felicity Plummer, manager at Italian restaurant Tutto Bene, said today she had had a text message from Antarctica New Zealand chief executive Lou Sanson after the incident, indicating Key had collapsed due to jet lag.

Plummer she said Key was looking ill when he arrived at the restaurant and thought at the time he would not make it through dinner.

The prime minister fainted as he was leaving the restaurant and was taken to Christchurch Hospital where he was checked by specialists and discharged two and a half hours later.

He had returned on Wednesday from a holiday in Hawaii and is now on a seven-hour flight to Antarctica.

Plummer said that when Key and his entourage of 10 arrived at the restaurant at 7.30pm, she thought he looked unwell and commented to a colleague that he looked tired.

"He looked extremely tired and he was perspiring."

After Key had finished his meal she saw him get up from his seat and his legs buckle.

As he was falling, someone next to him grabbed him under his armpits and tried to hold him up.

She said he appeared to lose consciousness for few seconds and seemed dazed when he came to.

"He didn't actually thump on the ground or anything," she said.

"He was assisted and helped to his feet."

The restaurant was fully booked and other diners gasped in shock, but they did not crowd him and had given his diplomatic protection officers room to take care of him.

"I felt sorry for him – he just looked absolutely exhausted," she said.

Key had been gracious throughout the incident.

"Poor man, I would think he would need to take a bit of a rest now."

Restaurant owner Paulette Mannucci said it was one of the most bizarre incidents that had happened in the restaurant since she had owned it.

It was the Prime Minister's first visit to Tutto Bene and he was one of their most famous guests.

A spokeswoman for Key said he was seen by three specialists during his time at the hospital but they had not given a reason for the fainting.

"He was given a clean bill of health," his spokeswoman said.

"They said these things happen sometimes."

She said Key had not been unwell after returning from Hawaii.

A man who was dining at a table next to Key said the prime minister had finished his meal and had just got up to leave when he collapsed.

"I felt something hit the back of my chair and then people said he was collapsing," the diner said.

"A bunch of people surrounded him and they dragged him to chair and started fanning him with menus, trying to give him air, and they gave him water.

"He looked really pale and really sick. He just sat there and face-palmed for about 10 minutes and then he got up and people helped him out."

The diner said it was not overly hot and stuffy in the restaurant and Key had been relaxed and "animated" prior to collapsing. He had chatted freely with other diners, sharing the odd joke.

"I don't know if he was ever unconscious but he did collapse and he had to be dragged to his feet," he said.